This invention pertains to microwave components and more particularly is concerned with voltage tunable filters.
The need arises in many microwave applications for tunable filters. In a direct frequency synthesizer, for example, a mix-and-divide approach is used to synthesize discrete frequencies. The mixing process generates many unwanted high level spurious signals which must be filtered. Typically, four or more local oscillators are used, yielding several different frequency ranges to be filtered.
Prior to the tunable half wavelength microstrip bandpass filter, disclosed herein, approach, it was necessary to utilize either a single fixed filter or a bank of four or more fixed filters in which the correct filter could be switched in line with the mixer output. These filters would most likely be the coupled line type for frequency ranges above 1 GHz. The filter bank method has the disadvantage of requiring a lot of space, the amount of which is directly proportional to the number of filters required. Quarter wavelength coupled line filters with varactors are known. These filters are voltage tunable but do not have the bandpass characteristics needed for applications such as direct frequency synthesizers. Further, one end of each quarter wavelength line has to be at RF ground, which is difficult to define at high frequencies.
The lack of a single filter suitable for filtering unwanted mixer products has been one of the difficulties in the mix-and-divide, or direct synthesizer approach.
An object of the invention is to provide a bandpass filter voltage tunable over a frequency range. Another object of the invention is to provide a simple bias circuit to control the tuning voltage.